1. Field of the Invention
The present invention relates to a method for estimating the distance between a tracking type laser interferometer and a target, and a tracking type laser interferometer, and in particular, to a method for estimating the distance between a tracking type laser interferometer, which is capable of accurately estimating the absolute distance during measurement even in a case where the detection range of a sensor for detecting a deviation of an optical axis is narrow, and a tracking type laser interferometer adopting the estimation method.
2. Description of the Related Art
Such a tracking type laser interferometer has been known which is composed of a laser interferometer incorporating an optical axis deviation detection sensor for detecting the deviation of an optical axis of return light, a two-axis turning mechanism for turning the laser interferometer to any optional direction, and a retroreflector, which is fixed at an object to be measured (Japanese Published Unexamined Patent Application No. S63-231286, hereinafter called Patent Document 1, and Japanese Published Unexamined Patent Application No. 2007-57522, hereinafter called Patent Document 2). Here, the retroreflector is an optical element, in which incident light and reflected light become parallel to each other, and it is possible to execute interferometric length measurement in any optional direction by controlling the two-axis turning mechanism based on the output of an optical axis deviation detector sensor so that the deviation of the optical axis becomes zero (0).
A general tracking type laser interferometer is composed to include, as shown in FIG. 1, a measurement head 103 having a laser interferometer (hereinafter merely also called an interferometer) 101 and an optical axis deviation detection sensor 102 for detecting a deviation in the optical axis of measurement light and reflected light, a two-axis turning mechanism 104 for turning the measurement head 103 to any optional direction, an angle sensor 105 for detecting a turning angle of the two-axis turning mechanism 104, a retroreflector 107 fixed on an object 106 to be measured, and a controller 108 for tracking the object 106 to be measured and collecting measurement data.
The controller 108 collects a distance signal from the interferometer 101, a deviation of an optical axis from the optical axis deviation sensor 102, and an angular signal from the two-axis turning mechanism 104, and drives the two-axis turning mechanism 104 so that the deviation of an optical axis becomes zero (0).
Distance measurement using the tracking type laser interferometer is carried out as follows. FIG. 2 shows one example of the positional relationship among the optical axis of the laser interferometer 101, turning center O of the interferometer 101, and center P of the retroreflector 107 during tracking control. Here, for simplification, only one axis, turning in the horizontal direction, of the two-axis turning mechanism 104 is taken into consideration.
It is assumed that the turning angle in the horizontal direction is θt where the optical axis of the tracking type laser interferometer is expressed in a spherical coordinate system, and the turning angle in the horizontal direction is θr where the center of the retroreflector 107 is made into P, and the position P is expressed in the spherical coordinate system.
FIG. 3 shows one example of positional relationship between the measurement light of the laser interferometer 101 and points O and P when the tracking type laser interferometer is tracking the retroreflector 107. If the direction of measurement light to the center P of the retroreflector 107 positioned at distance L from the point O in the direction θr is θt, the following expression can be established between the axial gap Δθd for angles θr and θd (direction of the center P of the retroreflector 107), deviation d of the optical axis, and distance (target distance) L between the points O and P.d=2L sin(θr−θt)=2L sin Δθ2  (1)
If the angular deviation Δθd is obtained by transforming the expression, the following expression can be brought about.Δθd=(θr−θt)=sin−1(d/2L)  (2)
FIG. 4 is a block diagram of a general angular control system. Angular deviation Δθd is taken from the target value θr and the present value θt detected by an angular sensor 403, and is multiplied by an angular compensation element 401 to obtain an angular velocity instruction value ω. The angular velocity instruction value ω is input in an object 402 to be controlled.
In the tracking type laser interferometer, the two-axis turning mechanism 104 is controlled by adopting a feedback control system that uses or, which expresses the movement amount of the retroreflector 107 in the spherical coordinate system, as an instruction value, and uses an angle θt obtained by the angle sensor 105 incorporated in the two-axis turning mechanism 104 as a feed back signal. However, since in actuality, there is no method to observe θr, as shown in FIG. 5 using the conversion element 503 that converts the deviation d of an optical axis and distance L to the angular deviation Δθd, the two-axis turning mechanism 104 is controlled based on Δθd, which is obtained based on L and d in the expression (2), as the angular deviation.
As described above, it becomes necessary to obtain the distance L between the turning center O of interferometer and the center P of the retroreflector in the tracking control system. However, since the interferometer 101 measures a relative distance, the absolute distance L cannot be obtained. Since the resolution of millimetric order is sufficient for the distance L calculated in the tracking control, another method for estimating the distance L is prepared separately from the interferometer 101.
As the method for estimating the distance L between the turning center O of interferometer and the center P of the retroreflector, a method for using an absolute distance sensor and a method for measuring the distance of a point, which is already known, and obtaining the distance L using the distance from the point may be considered. However, the former one causes an increase in the entire system, and the latter one causes a decrease in the throughput for measurement.
In order to solve such problems, the applicant has proposed in Japanese Published Unexamined Patent Application No. 2007-309677 (hereinafter called Patent Document 3) that the distance is obtained based on an angular width corresponding to the width of a detection range by scanning an optical axis in the detection range of a light spot position detection element secured in the optical axis deviation detection sensor 102 using the angle sensor 105 incorporated in the drive mechanism.
According to the method proposed by Patent Document 3, although it is possible to estimate the absolute distance at an optional position, there is a problem that the narrower the detection range of the optical axis deviation detection sensor is, the greater the errors become.